Transducers which translate nonelectrical quantities into electrical signals have long been known. These devices are designed to provide a usable output in response to a particular physical quantity, property or condition which is desired to be measured. The term `sense element` has been used to define both the overall transducer as well as the transducer element which performs the first step in a multi-step translation process. For the purposes of the instant invention, the latter definition will govern.
Transducers with sense elements used to measure acceleration are known as accelerometers. Accelerometer applications in the automotive industry include incorporation into crash sensors for air bag deployment and ride motion sensors for active suspension components. Accelerometers also find use in the military and aerospace industries as rate of climb indicators and as components in smart weapons and missile arming devices.
Prior art sense elements have had difficulty in obtaining and maintaining the strength and response characteristics required by the rigorous working environment of such accelerometers. These sense elements relied heavily on cantilevered beams, seismic masses, and cylinder/ball arrangements affected by fluid pressure and traveling friction. These devices suffered from extreme fragility and breakage problems. In addition, it has often been difficult to make these prior art sense elements efficiently and economically, due to the rigors of large scale manufacturing processes. Finally, these devices often exhibit excess and undesired sensitivity in orthogonal axes.
U.S. Pat. No. 4,736,629 issued to Cole, (the U.S. Pat. No. '629 patent), and hereby incorporated by reference, discloses an accelerometer having a sense element comprised of a metallic upper plate having an internal opening surrounding a pedestal mounted to a semiconductor substrate. The pedestal is connected to the metallic plate by a pair of torsion members extending in opposite directions from the pedestal to the metallic plate. Fixed plates positioned on the semiconductor substrate correspond to portions of the metallic plate to form first and second capacitors. In response to acceleration normal to the substrate the metallic plate rotates around the flexure axis defined by the torsion members to vary the capacitance of the first and second capacitors.
The U.S. Pat. No. '629 patent expressly requires that the rotatable upper plate be made of metal and the substrate be a semiconductive material such as silicon. Although Cole teaches that the electrically conductive substrate will be covered with an insulating layer, the conductive nature of the substrate renders the disclosed sense element vulnerable to parasitic capacitance loss. As a result, Cole's sense element is potentially less accurate and less sensitive.
Cole also teaches that forming inertial elements from low density materials like silicon or silicon dioxide results in sense elements with less sensitivity to acceleration as compared with corresponding sense elements with inertial elements made of metal. The disclosure teaches that such semiconductive constructions are nonfeasible. Although admitting that the coefficient of thermal expansion of metal is significantly greater than that of silicon, Cole postulates that the structure of the U.S. Pat. No. '629 accelerometer sense element eliminates any bias due to differential thermal expansion or contraction. However, Cole fails to account for the effect of undesirable compressive stress induced in the torsion arms as a result of differential thermal expansion between the supporting pedestal and the underlying substrate.
In addition to undesirable stresses in the torsion arms, Cole's metallic upper plate is highly vulnerable to hysteresis and creep as well as plastic deformation from mechanical stresses. It will be appreciated that hysteresis defines lags or delays between the onset of acceleration and its resulting effect, i.e. the change in the relative capacitances, is highly undesirable in a sense element intended for use in applications requiring a high degree of accuracy and reliability, i.e. crash sensors for air bag deployment.
Even more importantly, Cole teaches that the sense element should be critically damped. However, this results in a sense element with a mechanical sensitivity which decreases the device's shock survivability. It will be appreciated that these characteristics make the use of a critically damped sense element highly undesirable in a crash sensor since such devices may be responsive to undesired signals.
It is thus an object of the present invention to achieve a sense element which solves the problems of the prior art, particularly with respect to the strength, survivability and response characteristics required of sense elements for accelerometers in general and particularly those used in automotive crash sensors for air bag deployment.
It is another object of the instant invention to provide a sense element having a measurement based width and a natural frequency and having incorporated therein a plurality of openings for modifying the natural frequency and the frequency response so as to produce a sense element having increased shock survivability.
Finally, it is also an object of the instant invention to provide a sense elements which has a structure and composition which substantially eliminates undesirable stresses resulting from differential thermal expansions or contractions and is free from hysteresis.